The present invention finds a particularly advantageous, although in no way limiting, application in ultra narrow band wireless communication systems. By ultra narrow band (UNB) is meant that the instantaneous frequency spectrum of the radio signals transmitted by the terminals to the access network has a frequency width less than one kilohertz.
UNB wireless communication systems of this kind are particularly suitable for M2M (machine-to-machine) and internet of things (IoT) type applications.
In a UNB wireless communication system of this kind the exchanges of data are essentially one-way on an uplink between terminals and an access network of said system.
The terminals transmit uplink messages that are collected by base stations of the access network with no need for association beforehand with one or more base stations of the access network. In other words, the uplink messages transmitted by a terminal are not addressed to a specific base station of the access network and the terminal transmits its uplink messages on the assumption that they will be received by at least one base station. Arrangements of this kind are advantageous in that the terminal does not need to carry out regular measurements, which are notably voracious in terms of electrical consumption, to determine the most appropriate base station to receive its uplink messages. The complexity lies on the access network, which must be able to receive uplink messages possibly transmitted at arbitrary times and on arbitrary center frequencies. Each base station of the access network receives uplink messages from the various terminals within range.
A mode of operation of this kind, in which the exchanges of data are essentially one-way, is entirely satisfactory for numerous applications, such as remote reading of gas, water and electricity meters, remote surveillance of buildings or houses, etc.
However, in some applications it can be advantageous also to be able to exchange data in the other direction, namely on a downlink from the access network to the terminals.
In particular it may be advantageous to transmit broadcast or multicast signals to the terminals. In particular, a plurality of frequency bands are possible for the transmission of the uplink messages, for example associated with respective different geographical regions that may have different regulatory constraints. The transmission of broadcast signals could then enable the terminals to identify the frequency band of the uplink in the geographical region in which they are located before transmitting uplink messages in a frequency band not intended for this purpose. For example, it would be possible to transmit the broadcast signals in the frequency band of the uplink or in a frequency band having a predefined frequency difference relative to the frequency band of the uplink.
Broadcast signals of this kind could equally be used to transmit any type of information that could be useful for all the terminals or for a large number of them.
However, to limit the production cost of the terminals the detection of the broadcast signals must be carried out in a simple manner that is economical from the electrical consumption point of view.
Moreover, the broadcast signals must be transmitted with limited impact on the collection of the uplink messages. In particular, to reduce the deployment cost of the access network, the use of half duplex base stations may be envisaged, i.e. base stations that can receive uplink messages and transmit broadcast signals, but not both at the same time. In this case, a base station that is transmitting a broadcast signal is not available to receive uplink messages transmitted by the terminals and so uplink messages may be missed.